New fossils described in the journal Nature this week seem to close the door on a controversy that has raged for 40 years. They also confirm that the beginnings of the human genus more than 2m years ago began with a burst of biodiversity. These fossils place yet another nail into the coffin of the view we evolved in a gradual, step-like, progression, instead confirming the bushy nature of our evolutionary past.

Four big bursts

The 7m-year-or-so history of human evolution is characterised by four big bursts of biodiversity.

Digital drawing of Sahelanthropus.Sisyphos23

The first burst happened not long after we shared an evolutionary common ancestor with living chimpanzees some 7m years ago. This one produced the three very earliest two-footed ape (or hominin) groups unearthed by anthropologists in the last decade or so: Sahelanthropus from Chad, Orrorin recovered in Kenya and Ardipithecus from Ethiopia.

These groups hint at great diversity among the very early hominins and we can expect many more spectacular discoveries to be made in the future from around this murky time in our biological history.

The second big burst happened from around 4.5 million years ago and heralded the arrival of the well-known hominin Australopithecus (or southern ape). It was described in the 1920s by Australian anatomist Raymond Dart for the skull of the Taung child in South Africa. It was eventually extended to include famous specimens such as the Lucy skeleton from Ethiopia.

Replication of Australopithecus afarensis.Rafaelamonteiro80

Today we recognise four species of Australopithecus – anamensis, afarensis, africanus and sediba. The last of them was found by South African scientists only a couple of years ago. Together, they span roughly 4.5m to a little less than 2m years ago.

The southern ape is famous also for its role in fulfilling Darwin’s prediction that the heartland of human evolution was Africa, where gorillas and chimpanzees reside today.

The last two biodiversity bursts occurred coincidentally around 2.5m years ago and marked the arrival of Paranthropus and our own genus Homo.

A large-toothed, ecological specialist, Paranthropus seems to have disappeared by about 1m years ago, while Homo, of course, survives as us until today.

Paranthropus was made famous by Louis and Mary Leakey’s 1959 nut-cracker man. The genus contains three species – aethiopicus, boisei and robustus – inhabiting the savannas of both eastern and southern Africa.

Homo is by far the most diverse of all hominins we know about. Our genus contained at least ten species including we the sole-surviving and youngest, Homo sapiens.

A controversy settled?

Endocast of a Homo rudolfensis skull.Tim Evanson

The three new fossils announced by Meave Leakey and her team in Nature belong to perhaps the earliest member of Homo. Yet the species in question, Homo rudolfensis, has had a rocky road to acceptance.

For the 20 years since the first member of this species – cranium KNM-ER 1470 – was found on the eastern shores of Lake Rudolf (now Turkana) in northwest Kenya, views have varied enormously about its place in the human evolutionary tree.

The bony features of 1470 are unusual. It has a tall and very flat face, with a very large upper jaw and tooth sockets, and a large braincase indicating a large brain for this evolutionary time. It differs in quite dramatic ways from the much smaller, less rugged, and smaller toothed fossils we have for habilis.

Replica skull of homo habilis.120

One of the big problems for scientists wanting to test the two species hypothesis was that 1470 lacked teeth and a lower jaw. This was important because habilis was described using a lower jaw with intact teeth from Olduvai Gorge in Tanzania.

The so-called Type for habilis was the mandible designated OH 7 (Olduvai Hominin 7), and to do a proper comparison requires comparing to the Type. Also, OH 7 lacks a face and braincase, precluding comparisons with these parts of 1470.

A new piece of the puzzle

The new fossils dated between nearly 2m and 1.8m years ago provide us with the first extra face fragment in 40 years that looks a lot like the face of 1470. Importantly, its jaw contains teeth. Also found were two lower jaws, and they would seem to provide a very good match for both 1470 and the new cranium, KNM-ER 60000.

It would seem that as of today rudolfensis has finally earned its rightful place in the human evolutionary tree.

The fact that the same and very distinctive anatomical pattern is seen over such a long period in several fossils confirms the distinction of rudolfensis from habilis. And the differences in features of the face, braincase, jaws and teeth between them are much more than we would anticipate from sex alone.

Twigs in an evolutionary bush

It is truly remarkable that some anthropologists and plenty of creationists still persist with the idea that human evolution unfolded in a step-like fashion. That our evolution can be thought of as a ladder, each step getting closer to humans, as if following some preordained plan.

The fossil record demonstrates powerfully that human evolution proceeded in four major biodiversity bursts and that our twig in the bush is only one of many. We are the most recent member of Homo to appear, and today we are alone.

Yet, from the very beginning of the human genus, diversity was the rule, as these new fossils from Kenya once again confirm.